1. Field of Invention
This invention relates generally to the design of electrical machines, and more particularly to the design of alternators, generators, and motors having low inductance in the armature circuits.
2. Description of the Prior Art
As is well understood by those skilled in the art, electrical machines have an internal impedance that interacts with other system impedance to determine the performance of the combined system. In a motor, the inductance is that portion of the internal impedance related to magnetic energy storage within the electrical machine as it is energized to deliver mechanical work. The electrical system driving the motor must deliver the energy to be stored in the inductor in addition to the energy for the mechanical work to be performed. This necessitates increases in the capacity of generators, wiring and transformers needed to supply the motor.
In alternators and generators the internal impedance is, perhaps, even more important. The alternator or generator impedance combines with the load impedance to determine the performance of the whole system. As the internal impedance of an alternator or generator is made to be a smaller fraction of the total impedance, the output voltage of the alternator or generator becomes a larger fraction of the ideal (pre-loss) voltage provided by the idealized source. In the current art care is generally taken to provide low resistance pathways in the copper windings of an alternator or a generator in order to minimize internal resistance and to minimize the power lost in the alternator or the generator and the waste heat that needs to be dissipated.
Another factor in the impedance of the alternator or generator is the inductance of the output windings. This inductance is a direct result of winding the output coils around magnetic pathways in the alternator or generator, this being the technique usually used to generate the output voltage. Any output current in such windings will store magnetic energy in the same magnetic pathways, as is well understood. The inductance, "L" of the circuit is related to this stored energy by the equation EQU L=2*(Energy Stored)/(Current.sup.2)
The inductance of the output windings is part of the internal impedance and acts to filter the output voltage applied to the load. As frequencies get higher this inductive impedance blocks an increasing proportion of the ideal voltage provided by the alternator or generator and prevents it from acting on the load. While this has not been much of an issue for 60 Hz synchronous generators, it becomes a substantial design challenge for high frequency alternators. This has been known for some time; for example Griffing and Glockler present the design of a "High Frequency Low Inductance Generator" in U.S Pat. No. 3858071.
High frequency alternators or generators are desirable in that high levels of output power can be achieved with physically small magnetic paths, resulting in physically compact units. Claw pole alternators are typical of the physical design of high frequency generator devices and achieve high frequency by having a plurality of alternating poles. A disadvantage of these physically small claw pole units is that the close proximity of multiple poles and multiple magnetic pathways allows for unnecessary storage of substantial amounts of magnetic energy, resulting in high output inductance.
High output inductance causes several difficulties in the operation of high frequency alternators or generators. The impedance, Z of the inductor grows directly with the operating frequency (.omega., rad/sec) as shown in the following formula:
Z(.omega.)=j*.omega.*L(j=imaginary operator)
The higher the frequency, the greater the impedance and filtering. To overcome this filtering, the ideal voltage must be increased as the frequency is increased. The ideal (pre-loss) voltage is usually increased by increasing the magnetic excitation level of the field, leading to higher magnetic intensity levels in the magnetic pathways. Since core losses due to eddy current generation are proportionate to both the frequency squared and the magnetic intensity level squared it will be understood that the need for extra excitation to overcome the inductive impedance of the output will lead to high core losses at high frequency operation. At the limit when the excitation levels reach the point where magnetic pathways become saturated, further .excitation is precluded, and the output of the device drops off with further increases in operating frequency.
As a counterpoint to this, if internal inductance were negligible, then the output voltage would rise with increasing frequency due to the increased change of flux with time. The excitation levels could then be reduced as the frequency increased, leading the device away from saturation. The reduction of core loss due to the decrease in excitation would offset the expected increase in core loss due to the increase in frequency such that the core losses would remain nearly constant with operating frequency.
It is therefore an objective of this invention to provide an electrical machine that may be used as a high frequency alternator with low output inductance.
Furthermore, high frequency alternators are often poly-phased devices used with solid state circuits to rectify, switch, commutate or chop the output and reform it into DC or desired power frequency (50 or 60 Hz, etc) AC forms. In such devices individual alternator output phases are turned on and off at high frequencies, again invoking the filtering of the output inductance. Also, it is common for the output inductance of one phase to be linked by mutual inductance to the output of other phases so that the sudden change in current (switching) in one phase produces unwanted voltage transients in the other phases.
It is therefore a further objective of this invention to provide an electrical machine that may be used as a poly-phase high frequency alternator with minimal adverse effects caused by mutual inductance between phases.
It is often desirable for the output voltage of a high frequency alternator to be controlled independently of its rotational speed. This is usually accomplished by the use of a field coil that allows an externally applied electrical current to control the level of magnetic excitation within the alternator. The field coil magnetic circuit provides a pathway for the storage of large amounts of magnetic energy and contributes to inductance of the output circuits.
It is well known in the art that small air gap lengths between the rotor and armature reduce the proportion of fringing effects of flux passing between the rotor poles and the armature circuit. Reducing the fringing effect of this flux is important for controlling the voltage waveform and efficiency of an electrical machine. Small air gaps also reduce the required field excitation level and the attendant energy losses as well as leakage flux levels. However, small air gaps increase the amount of magnetic energy which the output circuits store in the field coil magnetic circuit and thus increase the output inductance of the machine.
Permanent magnet generators and alternators avoid this problem of the field coil magnetic circuit contributing to the output circuit inductance because the magnets themselves are high reluctance elements and limit the magnetic energy that can be stored by the currents in the output circuits. However, permanent magnet machines do not provide for control of the output voltage independently of the rotational speed.
As noted for the typical high frequency alternators, such as the claw pole type, the close proximity of multiple poles and magnetic paths gives rise to the unnecessary storage of large amounts of magnetic energy. This is important in the field excitation circuit as well as the output circuit because of the saturation and core-loss issues already mentioned. It should be noted that the majority of magnetic energy is stored in the high reluctance air spaces that are interconnected by the low reluctance ferro-magnetic pathways in which the saturation and core-losses phenomenon occur. So called "leakage flux" passes through the air spaces to complete magnetic circuits without going through the intended pathways that link output coils. In physically compact machines where many poles and pathways are arranged in close proximity the leakage flux can become a high percentage of flux, making the machine inefficient.
It is therefore an objective of this present invention to provide a low inductance electrical machine with output voltage which can be controlled independently of the rotational speed, and with small air gaps between the rotor and the armature in order to promote efficiency and a minimum of fringing leakage flux.
Electrical machines based on armatures with poloidal windings around a stator shaped as an annular ring have long been know. Kirkley and Smith present a generator design based on radial air gaps in U.S. Pat. No. 4,087,711. Langley and Fisher disclose a DC motor based on this configuration in U.S. Pat. No. 4,547,713. Further improvements were presented by Radovsky in U.S. Pat. No. 5,798,594 in which a brushless synchronous machine is presented with axial air gaps completing the magnetic circuit through an annular ring stator in a fashion that greatly limits the leakage flux from the field. The rotors in these designs are relatively complex, present difficulties in establishment of air gap clearances, and do not address the issues of output inductance or mutual inductance between phases.
It is therefore a further objective of this invention to provide a low inductance electric machine which may be used as a brushless alternator with low leakage of the field flux combined with simplified rotor construction and provision for independently establishing multiple air gaps.
Recent work by Groehl, disclosed in U.S. Pat. No. 5,565,836, presents methods for achieving the nullification of unnecessary components of flux within a toroidally wound inductor by use of concentric windings around the arcuate axis of the toroid combined with electrical connection of such windings so that currents of equal magnitude and opposite direction provide flux cancellation. The cancellation of internal flux fields within a toroid or annular ring can be used to create low inductance, and is fundamental to the design and operation of the common electrical power transformer. It is therefore a further objective of this invention to provide an electrical machine wherein the benefits of flux cancellation within an annular ring, or other closed shape, can be combined with arrangements for the efficient transformation between mechanical and electrical energy.
The previously mentioned high frequency alternators may be used for the production of (low) power frequency (60 Hz) AC power through methods disclosed by Hilgendorf in U.S. Pat. No. 3,916,284 and improvements presented by Tupper in U.S. Pat. No. 6,051,959, "Apparatus for Resonant Excitation of High Frequency Alternator Field". In this use the field excitation of the high frequency alternator is subjected to 60 Hz amplitude modulation. This leads to 60 Hz fluctuations of the magnetic field throughout the alternator's magnetic core, with attendant possibilities for eddy current core losses. Many traditional alternators are designed for essentially constant levels of magnetic excitation. These traditional alternators typically use core structures, such as solid iron rotors, that are not optimized to reduce eddy current losses. For constant levels of excitation this is acceptable, as there is little change in field excitation and therefore little core loss. These traditional alternators are not suited to use with 60 Hz amplitude modulation of the field; the core losses due to field modulation would be too large.
It is therefore a further objective of this invention to provide an electrical machine wherein the field excitation may be amplitude modulated at power frequencies while core losses are minimized.
As is well known, many electromechanical devices can be run in either a motor or generator mode. A generator with low internal inductance might also be operated as a motor with low internal inductance. In motor operation, low internal inductance reduces the requirements for the electric supply system to handle energy that is stored in the magnetic field of the device. Furthermore high armature inductance can impede the rapid change of armature pole currents and magnetic fields, thereby restricting the high frequency response of the motor. A motor with low armature inductance would allow relatively high frequency operation of the motor, which is useful in variable speed applications.
It is therefore a further objective of this invention to provide an electric machine that may be used as a motor with low inductance of the armature circuits.
In stepper motor operations precise control of the shaft position is achieved through the creation of a discrete step relationship between rotor pole position and armature excitation. This is useful for many industrial applications requiring careful control of shaft position. It is therefore a further objective of this invention to provide for an electrical machine with low armature inductance and which may be operated like a stepper motor.
In contrast to the stepper motor operations, during synchronous motor operations it is desirable to achieve a smooth rotation of the shaft. It is an additional objective of this invention to provide a low inductance electric machine that may be used as a synchronous motor with smooth rotation of the shaft.
Where motors are used in systems with requirements for regenerative braking it is desirable to be able to control the regenerated voltage and current independently from shaft RPM. For example, an electrically powered automobile using regenerative braking to stop at a traffic light would need to control the generated power so that it was constantly suitable for recharging the vehicle battery even as the vehicle slows to a stop.
The rotor inertia of a motor represents an energy storage mechanism on the mechanical side of motor operation. Particularly for control situations, such as with stepper motors, it is desirable to minimize mechanical inertia in the motor. It is therefore an additional objective of this invention to select features that reduce the mechanical inertia of the rotor.